I've just tested OPC guest checkouts on my testshop with Paypal express, and with Square (creating a new App for the testshop), works wonderfully.
I have a question about the Paypal setup though, could not find any mention of this in the documentation.
For standard 3-page checkout processing, the Paypal return URL is supposed (I think) to be set to:
https://www.mysite.com/MY_SHOP/index...eckout_process
For OPC, is this supposed to be kept as is, or better changed to something specific to OPC?